Synthetic paper structures of aromatic polyamides

ABSTRACT

NONWOVEN, FLEXIBLE SHEET STRUCTURES OF COMMINGLED FIBRIDS OF A NONFUSIBLE AROMATIC POLYAMIDE AND SHORT AROMATIC POLYAMIDE FIBERS WHEREIN ALL FIBERS HAVE AN INITIAL MODULUS LESS THAN 80 GM./DENIER ARE DISCLOSED HEREIN. THE STRUCTURES HAVE GOOD THERMAL PROPERTIES, GOOD FLEX LIFE AND A HIGH DEGREE OF ELONGATION. THE STRUCTURES CAN BE PREPARED BY CONVENTIONAL PAPER-MAKING PROCEDURES. THE STRUCTURES MAY CONTAIN ADDITIVES TO INHIBIT OXDATION, ETC., AND BINDERS, IF DESIRED.

United States Patent 3,756,908 Patented Sept. 4, 1973 3,756,908 SYNTHETIC PAPER STRUCTURES OF AROMATIC POLYAMIDES George Conrad Gross, Waynesboro, Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Feb. 26, 1971, Ser. No. 118,560

Int. Cl. D2111 5/12 U.S. Cl. 162-146 13 Claims ABSTRACT OF THE DISCLOSURE Nonwoven, flexible sheet structures of commingled fibrids of a nonfusible aromatic polyamide and short aromatic polyamide fibers wherein all fibers have an initial modulus less than 80 gm./denier are disclosed herein. The structures have good thermal properties, good flex life and a high degree of elongation. The structures can be prepared by conventional paper-making procedures. The structures may contain additives to inhibit oxidation, etc., and binders, if desired.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to synthetic nonwoven sheet structures. More particularly, this invention is directed to papers prepared from synthetic organic polymeric materials.

(2) Description of the prior art Nonwoven sheet structures, such as papers, are known in the art, as for example, those described in Morgan U.S. Patent 2,999,788. This patent describes the preparation of fibrids of various polymers and their use in making synthetic papers. It is also known in the art that fibrids of nonfusible, aromatic polyamides can be used to prepare papers which have good thermal properties, i.e., they resist degradation at elevated temperatures, and have good electrical insulative properties. Thus, such papers find usefulness as electrical insulation where high temperatures are commonly encountered. It has also been found beneficial for such use to combine the fibrids with short highly crystalline fibrids of the same aromatic polyamide used to prepare the fibrids.

The above-described papers are usually calendered at high temperatures (ordinarily above the second order transition temperature of the polymer) and pressures to improve their mechanical properties by increased bond strength. But although the calendering operation produces papers with good tensile strength and elongation, it also increases their brittleness, which consequently decreases their flex life and conformability. Furthermore, when less severe calendering conditions are employed to avoid the increase in brittleness, and thus increase flex life, the resulting papers are found to have a deleterious loss in elongation-to-break.

The papers of this invention (which employ aromatic polyamide fibers of low modulus), however, have good elongation-to-break along with a low degree of brittleness, i.e., a good flex life.

SUMMARY OF THE INVENTION The invention is directed to a flexible sheet structure consisting essentially of a nonwoven, nonfused, commingled mixture of (1) about 15 to 90 percent by weight of fibrids of a nonfusible aromatic polyamide, and (2) about to 85 percent by weight of short fibers of a nonfusible aromatic polyamide having an initial modulus less than 80 grn./denier. These structures have good thermal properties, i.e., resist degradation at high temperatures, and possess a good flex life, good elongation-to-break and good conformability.

In a preferred embodiment, the short aromatic polyamide fibers employed should have an initial modulus of between 20 and 80, a denier of 0.5 to 5.0, and an elongation of 20-65%, and the sheet containing them should be compressed to a thickness of between about 1 to 30 mils.

These sheets are primarily of interest for use as electrical, insulative materials at elevated temperatures. For the most part, the dielectric properties of such mixed sheets are contributed by the fibrids and for best results the weight ratio of fibrids to fibers should be at least 1:1 and preferably at least 12:1 for electrical grade papers.

DESCRIPTION OF THE INVENTION The sheets of this invention are paper-like and can be prepared using conventional paper-making processes and equipment. Thus, the fibrous materials, i.e., the fibrids and short fibers, can be slurried together to form a mix, which can be converted to paper on a Fourdrinier machine, or other means, such as a handsheet mold containing a forming screen, may be used.

Fibrids are small, nongranular, nonrigid fibrous or film-like particles. Two of their three dimensions are on the order of microns. Their smallness and suppleness allows them to be deposited in physically entwined configurations such as are commonly found in papers made from wood pulps. The fibrids used in the sheets of this invention can be prepared by precipitating a solution of the aromatic polyamide in a fibridating apparatus of the type disclosed in U.S. Patent 3,018,091, where the polymer is sheared as it precipitates.

'For use in this invention, the fibrids and the short fibers are prepared from nonfusible aromatic polyamides. The term aromatic polyamide as used herein is defined as a polymer wherein the amide group, i.e., the

i l-1tradical where R is hydrogen or alkyl of l-6 carbon atoms, of each repeating unit is linked through the nitrogen and the carbon atom to a carbon atom in the ring of separate aromatic ring radicals. The term aromatic ring is defined herein as a carbocyclic ring possessing resonance. The term non-fusible is defined as meaning that the polymer begins to decompose before it begins to melt.

The nonfusible aromatic polyamides may be prepared by reacting an aromatic diacid chloride with an aromatic diamine. The carboxyl groups of the diacid chloride and the amino groups of the diamine can be oriented ortho-, para-, or meta-relative to each other, with meta-orientation being preferred. The reaction is carried out at a low temperature, e.g., below C. Aromatic amino-acyl compounds may also be used. In addition, other polymerforming ingredients which need not contain an aromatic nucleus can be included in amounts of less than about 10 mol percent without detracting from the desired physical and chemical properties of the polymers used to prepare the fibrids and fibers. Moreover, substituents such as lower alkyl, lower alkoxy, halogen, nitro, low carbalkoxy, or other groups (e.g., sulfonate) which do not form a polyamide during polymerization, may be attached to the aromatic ring nuclei. Preferably, however, the diamine and diacid will be entirely aromatic, i.e., unsubstituted, re sulting in a polymer wherein the units linked by the amide group are wholly aromatic.

Representative examples of aromatic ring nuclei include and the like. Suitable polyamides are described in greater detail in US. Patent 3,094,511 and British Patent 1,106,190. A preferred aromatic polyamide is poly(metaphenylene isophthalamide). The polyamides employed will preferably have an inherent viscosity of about 1.2 to 2.0 when measured at 25 C. in N,N-dimethylacetamide containing four percent lithium chloride based on the weight of the solution, at a concentration of 0.5 gram of polyamide per 100 cc. of solution.

Once the fibrids are prepared they are dispersed in an aqueous medium to form a slurry for use in making the sheets of this invention. Preferably, the slurry will contain less than about one (1) percent by weight of the fibrids.

The low modulus aromatic polyamide fibers used in this invention are short fibers commonly referred to as floc. Preferably, the polyamide floc will comprise fibers less than one inch (2.54 centimeters) in length with the optimum length being about 0.25 inch (0.6 centimeter). The modulus of these fibers will be dependent primarily on the amount of draw given to the continuous filament form of the fibers and on whether or not they have been exposed to high temperatures, i.e., temperatures in excess of 300 C., prior to use. In general, filaments drawn at low draw ratios, e.g., in the range of 3.0x to 4.7x, can be exposed to higher temperatures with less increase in their modulus values. Accordingly, fibers drawn at high draw ratios, e.g., 4.8 or above, should not be passed over crystallizer rolls and the like so as to insure that their modulus remains less than 80. Appropriate yarns or tows of the polyamide are cut to the desired fioc length by any suitable manner, e.g., by use of a helical saw cutter, and then the floc is dispersed in an aqueous medium to form a slurry for use in making the sheets of this invention. Preferably, the slurry will contain less than one percent, by weight, of the floc.

Suitable fibers are those having a denier of from about 0.5 up to 10 or more. The higher deniers are not as readily bonded as are the smaller fibers, and deniers, less than about are preferred. Most preferred are fibers having a denier between about 1 and about 3.

To prepare the flexible, sheet-like structures of this invention, the slurries of the fibrids and fibers are mxied in preselected amounts to form a paper-making stock that can be added to the headbox of the paper machine where it is spread uniformly on the paper-forming wire. Water drains from the stock as it moves along and drainage is assisted by conventional means, e.g., vacuum, as a sheetlike mat is formed on the wire. The wet sheet, which may contain a weight ratio of paper solids to water of from 1 to 4 to 1 to 8, leaves the wire and enters the drying section of the machine. The function of the dryer section is to remove the water without damaging or destroying the sheet. Drying is carried out by passing the wet sheet in contact with the surfaces of steam-heated dryer cans. The drying cans, of which there may be as many as 40 or more in two rows, are grouped into sections with each section being independently driven to allow some draw of the sheet while it is being dried. The draw will vary depending on conditions but the overall draw will usually be on the order of about 5 percent. For better contact, the sheet is held against the undersides of the bottom cans by porous dryer felts. The steam pressure in the dryer cans can range from 30 to 150 pounds per square inch (2.1 and 10.5 kilograms per square centimeter) gauge and increases along the machine. The paper will reach a maximum temperature not exceeding 185 C. piror to leaving the machine. Under common processing conditions, the papers will contain from about 2 to about 10 percent water, based on the weight of the moist paper.

If desired, the wet web of papers so prepared can be calendered, for generally, subjecting the papers to high temperatures and pressures will improve their physical properties due to the increased bonding strength arising from compaction. However, calendering conditions must not be such as to cause extensive crystallization of the fioc fibers in the paper. Calendering pressures may range from about 200 to about 900 pounds per inch (35.8 to 101.1 kilograms per centimeter). Although room temperature calendering will provide papers with suflicient strength for some applications, the calender temperature normally should be at least 220 C., and the maximum calender temperature will vary depending on the balence of paper properties desired but, as previously indicated, it must not cause extensive crystallization. In general, the calender temperature will be in the range of 240 C. to 290 C. although temperatures as high as 325 C., or higher, can be used to provide a satisfactory result in some instances. In addition to greater elongation, calendered papers of this invention are typically smoother than papers obtained from crystallized fibers, especially for papers calendered at the higher temperatures. This smoother surface is especially desirable when the papers are used for making slot liners for electric motors becaue it facilitates liner insertion into the stator slots. Uncalendered papers also have high elongation, and for some uses the papers may be used in this less dense, more porous form.

Papers of this invention may, if desired, be prepared by ply-bonding two or more uncalendered sheets together during calendering to provide a laminated product. Adhesion between sheets of multi-ply papers is greatly improved by treating all bonding surfaces in a corona discharge, i.e., the soft, blue discharge which indicates the incomplete electrical breakdown of a gas at or near atmospheric pressure. This treatment is particularl useful for calendering, wet or dry, at temperatures less than 285 C. Corona treatment consists of running a paper sheet between a ground, usually a dielectric-coated steel roll, and a high voltage electrode. Such treatment and equipment is common to the film and cellulosic paper industries. For treating of materials in this invention, about 0.1 to 1.0 watt of electrical discharge is used for each foot (30.5 centimeters) per minute treating speed and inch (2.54 centimeters) of electrode width. The gap between the electrode and the roll is normally kept at about 3X sheet thickness.

If desired, an inhibitor may be added to the paper to provide resistance to oxidative degradation at elevated temperatures. Preferred inhibitors are oxides, hydroxides and nitrates of bismuth. An especially effective inhibitor is a hydroxide and nitrate of bismuth.

Binders, e.g., acrylic polymers or epoxy resins, and the like, may also be added to the paper, either to the stock slurries, prior to sheet formation, or by conventional size-press addition to the formed sheet.

The percent that the paper elongates when stressed to the break, referred to herein, for simplicity, as elongation is dependent in part on the elongation of the fibers. The fibers need to have an elongation of at least 20% and for the most useful products the elongation will be in the range of 30% to The elongation of the papers is believed also to be dependent, at least in part, on the initial modulus referred to herein simply as modulus, of the fibers and the modulus of the fiber must be less than and for more optimum results less than 65. conformable papers prepared from fibers having a modulus of from 20 to 60 are a highly preferred embodiment of this invention. Initial modulus is defined as the ratio of change in stress to change in strain in the initial straight line portion (i.e., before the yield point) of a stress-strain curve following removal of any crimp. The ratio is calculated from the stress expressed in force (grams) per unit of linear density (denier) of the original specimen, and the strain expressed as percent elongation, i.e.,

A stress (gm.) X 100 A strain (percent) Xdenier In other words, the lower the initial modulus, the lower the resistance to stretching. Papers having good elongation are particularly useful in applications where conformability is required. The term conformability means that the papers conform smoothly to the shape of the object they are wrapped around. Good conformability is related not only to high elongation, but also to high fold endurance and low tensile modulus. The superior conformability of the papers of this invention is particularly apparent in lightweight papers (i.e., papers whose thickness is less than 5 mil).

In the following examples, which illustrate the invention, all percentages are based on total weight unless otherwise specified. In measuring the properties of papers prepared in the examples, the following methods were used:

Thickness: ASTM D-374 Apparent density, basis Weight/ thickness: ASTM D-374 and TAPPI T-410 Tensile strength and elongation: ASTM D-822-61T Fold endurance: ASTM-D-2l7663T (after conditioning to 75 F., 53% R.H., using a Tinius-Olsen Fold Tester) EXAMPLE 1 This example illustrates the preparation of fibrids. fioc and papers, and shows the superiority of papers of this invention over similarly prepared papers of aromatic polyamide fibrids and high modulus crystalline aromatic polyamide fioc.

Preparation of fibrids Initial modulus A solution of poIy(meta-phenylene isophthalamide) containing 14il% polymer with a viscosity between 50 and 75 poise (and an inherent viscosity of about 1.6) is passed to a fibridator of the type disclosed in US 3,018,- 091. The polymer solution contains 77.5% dimethylacetamide (DMAc), 2% water and 6.5% calcium chloride. The polymer solution is fed to the fibridator at approximately 120 pounds (54.6 kilograms) solids per hour. The polymer is precipitated using a precipitant composition controlled to contain between 30 and 40% DMAc, 58 and 68% water, and 2% calcium chloride. Precipitant flow rate to the fibridator is about 62.5 pounds per pound (28.4 kilograms per kilogram) of polymer solution. A rotor speed of about 7000 rpm. generates the shear required to produce good quality fibrids suitable for papermaking. The fibrids are washed with water to reduce the DMAc and chloride content to about 1.0% and 0.3%, based on polymer, respectively. The fibrids are subsequently refined to improve their filmy characteristics in a 36-2 Sprout-Waldron disc refiner at 0.8% consistency to a Schopper Riegler Freeness of 300 to 400 ml. Using the Clark Fiber Classification (TAPPI Standard T-233 su64), the fibrid size characterization is:

Screen size, mesh Percent retained 1.0 8 .0

Preparation of low modulus aromatic polyamide floc Total calcium chloride, and 30-l00% water based on dry polymer. The filaments are washed and drawn 5 X in an extraction-draw process in which the chloride and DMAc contents are reduced to about 0.10% and 0.5%, respectively. The filaments have a denier of 1.5 and typical properties are: initial modulus 32.4 grams/denier, elongation 33.4%, and tenacity 2.7 grams/denier. The filaments are then cut to floc length of 0.27 inch (0.68 cm.) and slurried in water to a concentration of about 0.23%.

Preparation of high-modulus crystalline aromatic polyamide floc The filaments, spun and drawn as described immediately above, are crystallized immediately after drawing by passing them over rolls at a temperature of about 340 C. A moderate additional draw in the crystallization step increases the total draw to about 5.0 X and produces filaments that have a denier of 2. Typical fiber properties are: initial modulus 94 gm./den., elongation 28%, and tenacity 4.5 gm./den. The filaments are cut to floc fibers having a length of 0.27 inch (0.68 centimeter). The cut filaments are slurried in water to provide a concentration of about 0.23% floc.

Preparation of paper The fibrids, at a concentration of about 0.6% in an aqueous medium are passed to a mixing T along with the slurry of low modulus aromatic polyamide fioc, at a concentration of about 0.23% in Water, at a weight ratio of fibrid to floc of about 1.5 to 1.0 (60% fibrids and 40% floc). The mixture is directed to the head box of a Fourdrinier paper-making machine and then to a forming wire for production of a wet sheet. The wet sheet is removed from the wire and passed to a dryer to reduce its moisture content by the use of steam-heated dryer cans heated to a maximum temperature of 137 C. The partially dried paper is then wound on a roll used as the supply for producing calendered papers.

A second paper was then prepared as described immediately above, except that the slurry of high modulus gm./den.) aromatic polyamide floc was used in place of the slurry of low modulus aromatic polyamide floc.

The basis weight of both papers is about 3.5 ounces per square yard. The papers produced are then dried, before being platen pressed at 240 C. and 1250 pounds per square inch (87.5 kilograms per square centimeter) gauge.

Properties of the papers before and after pressing are given in Table I.

*MD =Machine direction.

As can be clearly seen from the information in the table, the paper containing the low modulus aromatic polyamide fiber (i.e., the paper of this invention) has an elongation nearly twice that of the similar paper containing high modulus fibers. Papers having such a high level of elongation have good flex life and conformability.

When the dry, uncompressed papers are calendered at 15 feet (4.5 meters) per minute at a temperature of 280 C. and a nip pressure of 700 pounds per linear inch (125 kilograms per centimeter), the elongation of the paper containing the low modulus fibers is 10.2% versus only 7.0% for the paper containing the high modulus fibers.

EXAMPLE II TABLE II Paper contain- Paper containing high ing low modulus fibers modulus fibers Thickness, mils (10* em.) 8.3 (22) Basis weight, oz./yd. (gm/M 6.50 (220) Apparent density, gmJce 1.05 Tensile strength, MD", lh./in./oz./yd.

(grams force/enr/gm/M). 24.8 (130) 14.8 (78) Elongation, MD, percent 19. 2 17. 3 Fold endurance, 10 cycles. 4. 5 6.8

'MD =Maehine direction.

These results show that papers of the invention containing the low modulus fibers compacted at lower temperatures produce comparable elongation and significantly higher fold endurance levels than do papers containing high modulus fibers.

EXAMPLE III Papers are prepared in a manner similar to that discussed for Example I using the low-modulus fibers and the high-modulus control fibers. They are then dried completely in infrarred heaters before being calendered at 290 C. The paper containing the low-modulus fiber is calendered at feet (3 meters) per minute using a pressure of 750 pounds (338 kilograms) per linear inch and the paper containing the high-modulus fiber is calendered at feet (4.5 meters) per minute using a pressure of 600 pounds (270 kilograms) per linear inch. Some fiber crystallization probably occurs at this high temperature but the paper from the low-modulus fiber exhibits significantly better elongation than the control paper comprised of highly drawn and crystallized fibers. Results are given in Table III.

Aromatic polyamide fibers are spun as described in Example I. They are then drawn in the extraction-draw process and in the crystallization step (340 C.) to a total draw ratio of 4.70 to produce fibers having an initial modulus of 71. Higher modulus fibers are produced by using a 4.95 draw ratio and these fibers have an initial modulus of 94. The fibers have a denier of 2 after drawing and crystallization.

Papers are prepared from these fibers on a 31-inch (78.7-centimeter) Fourdrinier machine as described in Example I. They are dried and then calendered at 320 C. at 60 ft./min. and 600 pounds per linear inch. The paper prepared from the low-modulus fibers has a superior elongation. Results are given in Table IV.

TABLE IV Paper eontain- Paper containing g ing low modulus fibers modulus fibers Thickness, mils (10' cm.) 5.3 (14) 4.9 (13) Basis weight, oz./yd. (gm/M 3.6 (120) 3.5 (120) Apparent density, gm./cc 0.91 0.96

Tensile strength, lb./in./0z./yd. (grains iorce/cmJgmJM 20.2 (107) 19.5 (102) Elongation, percent 12. 5 19. 5

As earlier stated, the papers of this invention are useful in electrical applications as insulation where high temperatures are encountered, especially above 150 C. Thus, the papers are useful as electrical wire wrap, as motor slot liners, or as insulation in oil-filled transformers.

The preceding representative examples may be varied within the scope of the present total specification disclosure, as understood and practiced by one skilled in the art, to achieve essentially the same results.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described for obvious modification will occur to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A flexible sheet structure consisting essentially of a nonwoven, nonfused commingled mixture of about 15 to percent by weight of fibrids of a nonfusible aromatic polyamide, and about 10 to 85 percent by weight of short fibers of a nonfusible aromatic polyamide wherein all fibers have an initial modulus less than 80 gms./ denier, the total percent by weight being about percent.

2. The structure of claim 1 wherein both polyamides are wholly aromatic.

3. The structure of claim 2 wherein the aromatic polyamide short fibers have an initial modulus of less than 80 gms./denier.

4. The structure of claim 3 wherein the percent of fibrids present is at least 50% by weight.

5. The structure of claim 4 wherein the wholly aromatic polyamides are both poly(meta-phenylene isophthalamide).

6. The structure of claim 5 wherein the sheet is between about 1 mil and about 30 mils thick.

7. The structure of claim 1 wherein the aromatic polyamide short fibers have an initial modulus of between 20 and 60 gms./denier.

8. The structure of claim 7 wherein the percent of fibrids present is at least 50% by weight.

9. The structure of claim 8 wherein both polyamides are wholly aromatic.

'10. The structure of claim 9 wherein the sheet is between about 1 mil and about 30 mils thick.

.11. The structure of claim 1 wherein the sheet is between about 1 mil and about 30 mils thick.

12. The structure of claim 11 wherein the aromatic polyamide short fibers have an initial modulus of between about 20 and 60 gms./ denier.

13. The structure of claim 12 wherein the percent of fibrids present is at least 50% by weight.

References Cited UNITED STATES PATENTS 3,549,789 12/ 1970 Haroldson 162-146 2,999,788 9/1961 Morgan 2642l6 3,282,038 11/1966 Howell 162-146 S. LEON BASHORE, Primary Examiner W. F. SMITH, Assistant Examiner US. Cl. X.R. 162-457 R 

